1. Field of the Invention
The present invention relates to a prosthetic device for a lower leg or foot for use by a below-the-knee amputee.
2. Description of the Related Art
Although prosthetic devices have been in use for centuries, not until relatively recently have efforts been made to design prosthetic legs and feet to react dynamically to the cyclic loading and unloading of the foot during movement to simulate more closely natural gait. To this end, prosthetic devices typically are configured to store and release energy during normal body movements. Typical prosthetic feet include a spring plate arranged longitudinally approximately within the sole of a shoe. The spring plate is usually arranged so as to provide flexure of the foot while walking.
Materials undergo a certain amount of deformation when they are stressed. If a mechanical element experiences a steady deformation when acted upon by steady forces, it is exhibiting compliance which is the basic characteristic of a spring. A translational spring is a mechanical element which deforms by steady amounts when loaded by steady forces. A linear spring has a proportional relation between deformation and force. Thus, the spring stores work as energy associated with its deformation. This stored energy is called translational potential energy. Hence, the energy stored in the spring depends directly on the force transmitted to the spring. In other words, the work done by any force acting on a spring during a specified displacement is equal to the change in the kinetic energy of the spring. This statement is the basic work-energy principle of body dynamics in physics.
The single spring plate of the prior art may have varying thicknesses along its length to give varying compliances across the spring plate in order to simulate more closely the curling movement of a foot during a walking motion. Improvements to the spring plate have included multiple plates of different thicknesses attached or adhered to each other within the prosthetic foot in order to create different compliances along the length of the foot. However, these attempts to achieve different compliances by varying the thicknesses have not been successful because it has been found that spring plates are inherently limited in their simulation of the natural motion of a foot. In particular, as a prosthetic foot using such a spring plate is curled, as it is during a normal walking or running motion, the effort required to curl the prosthetic foot increases, thereby inhibiting the full range of motion. Therefore, it is desirable to provide a prosthetic device that provides a more natural simulation of the motion of a leg or a foot.